It would probably need to be added to the polar regions, if at all. It would be very irritating if your empire were suddenly cut in 2 by hell. It seems slightly odd for a land of fire and brimstone to replace the polar ice caps, but would be interesting (and it is a quite literal interpretation of Kyorlin's quite "out of the ice fire will come" ).
It could get really complicated if you use a map with both x wrapping and y wrapping (can this be done in Civ IV? I know it could in Civ III).
donut shaped worlds are fun, but tend to screw over the ai (a donut wraps in both directions: P)